Well, I would characterize the reason as having more to do with the fact that passing $a = 2 as an argument causes an alias to $a to be put into @_. Had a copy of $a's value been placed into @_ instead, then subsequently changing $a to 3 would not impact that copy.
print $a = 1, 0+$a, $a = 2, 0+$a, $a = 3, 0+$a;
# prints "313233"
The reason that $a = 2 passes an alias is because it is an "lvalue" expression, which just means that you can do previously mentioned things like ( $a = 2 ) = 3. And the reason that lvalue expressions cause aliases to be passed is two-fold: So you can modify arguments via code like $_[0] = 3 in your sub and because it is more efficient than creating a copy.
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